1. Field of the Invention
The invention relates to a transmission clutch that combines an alternative clutch pack design with a fingered piston actuator aligned with the clutch plates to reduce hot spotting problems peculiar to clutch designs using fingered piston actuators.
2. Background Art
In general, heat degrades a clutch's performance and accelerates the wear of friction materials and metal plates. Clutches with so-called finger pistons, which are fairly common in automotive transmissions, are particularly problematic.
A common type of multidisk clutch is shown in FIG. 1. The clutch includes a set of friction disks (three in the example shown) and a set of so-called reaction plates (two in the example shown). Each friction disk has a steel core with two layers of friction material secured to the two sides of it. Friction disks have splines or teeth at the inner perimeter, as shown in the figure, which mesh with an internal shaft or hub in a well known manner. Friction disks are axially moveable but rotationally fixed relative to the hub or shaft. In other words, each friction disk rotates together with the shaft or hub.
Reaction plates are disks usually made of steel or less usual, of cast iron. They have splines or teeth at the outer perimeter and mesh with a drum or a housing. Reaction plates are axially moveable but rotationally fixed to the drum or housing. Similarly, the piston and the apply plate, which bring the friction clutch into engagement by exerting axial force that compresses the pack of disks, are axially moveable and fixed to the drum or housing against relative rotation. The end plate is most typically axially fixed (although it may also be moveable) and fixed against relative rotation to the drum. The reaction plates, the piston, the apply plate, and the end plate rotate together with the drum (housing), while the friction disks rotate together with the shaft (hub).
In the example shown in FIG. 1, there are six sliding interfaces. Each sliding interface is created by a pair of surfaces which rotate relative to each other. More precisely, the sliding interface is defined by the surface of a part rotating with the shaft and the surface of a part rotating with the drum. In some clutches, the drum or housing is stationary, and this apparatus may be called a brake. However, both the shaft (hub) and drum (housing) are often rotating parts. If the clutch is disengaged, they rotate at different speeds. When the piston moves axially to engage the clutch, all the pairs of surfaces are brought into frictional contact. As a result, a frictional torque is generated which tends to slow down the parts rotating at higher speed and accelerate the parts rotating at lower speed. This process ends when the two speeds become equal and then the whole clutch rotates as a rigid body, without relative rotation of the shaft (hub) and the drum (housing). In the case of brake (a clutch with stationary housing), the engagement process is analogous and the shaft (hub) rotating at the beginning of the process is brought to stop as a result of engagement.
Arrangements of the pack other than that shown in FIG. 1 are possible. Each of the sliding interfaces is usually created by the pair of a friction material layer against a metal surface. Non-sliding interfaces, if there are such interfaces in the clutch, can be created by similar materials, e.g. steel against steel. The clutch shown in FIG. 1 does not have a non-sliding interface. FIG. 2 shows another common clutch design which has an additional reaction plate placed between the piston and the left outermost friction plate. The piston and the reaction plate next to it have outer splines (teeth) so that both mesh with the drum (housing), and therefore the interface between them is a non-sliding interface. Consequently, metal against metal contact is acceptable there.
Another double-sided clutch design, an inverted double-sided design, has been known for years in automotive and heavy machinery applications, which do not employ finger pistons. In that design, friction disks have splines at the outer perimeter and therefore mesh with the drum (housing). Consequently, the steel reaction plates have to be equipped with splines or teeth at the inner perimeter to mesh with the shaft (hub). The interface between the piston and the adjacent friction disk is a non-sliding interface. Similarly, the interface between the end plate and the adjacent friction disk is a non-sliding interface. As used in this description, double sided clutch pack refers to a clutch pack with at least one double-sided friction plate with a friction layer on both sides, regardless whether any single sided plates are included in the clutch pack.
The durability or period of warranty coverage may be limited as has been noted in some common double-sided friction plate clutches used in today's automotive transmissions, where the finger piston is used as shown in FIGS. 1 and 2. The finger piston is most typically employed when the hydraulic piston engaging the clutch is distant from the clutch pack. When the apply force is transmitted from the distant piston to the apply plate of the clutch by means of several axial “fingers”, an example of such a piston is shown in FIG. 3, the consequence of this design is that the axial force transmitted by the fingers is applied to the pack non-uniformly around the circumference. By contrast, in the designs with the piston having a circumferentially continuous side surface and acting directly on an apply plate, the force is applied uniformly around the circumference by the annular surface.